In general, optical films, such as a retardation film and a view angle compensating film, are disposed between a polarizing plate and liquid crystal cells, so that color change of liquid crystal display (LCD) is decreased and viewing angle is expanded to improve brightness. The optical film is mainly divided into two films: one stretched film prepared by stretching a polymer film to give optical anisotropy, and the other liquid crystal film prepared by coating a plastic substrate with a polymeric liquid crystal compound and curing the polymeric liquid crystal compound by UV irradiation. In particular, liquid crystal film may be mainly divided into a rod-type liquid crystal and a disc-type liquid crystal, depending on the shape of the liquid crystal molecules. Among them, the rod-type liquid crystal can be aligned in various shapes such as planar, homeotropic, tilted, splay, cholesteric shapes, and therefore their optical properties owing to the various shapes are also diverse and unique, compared to those of the stretched film. Thus, if the stretched film is directly coated with the polymeric liquid crystal compound to give the various liquid crystal alignment properties, the liquid crystal film may function as a protective film and an optical compensation film of the polarizer.
The liquid crystal film is generally manufactured by coating a plastic substrate with a composition for an alignment layer such as polyimide and polyvinyl alcohol to form an alignment layer, rubbing the alignment layer in a predetermined direction, and then coating the alignment layer with the polymeric liquid crystal compound. However, when such alignment layer is used, a liquid crystal film may be peeled off or shrunken from the alignment layer due to the insufficient adhesive force to the liquid crystal film under hot and humid environments. Moreover, when an alignment layer is manufactured using the rubbing process, there are several problems in that electrostatic discharge or defect sites may be caused due to contact with impurities during rubbing, and fine dust by a rubbing cloth may be generated.
To solve the above problems, liquid crystal alignment methods comprising a non-rubbing process have been developed. Among them, a photo-alignment is proposed, in which a liquid crystal alignment layer is manufactured by light irradiation. In this connection, examples of the photopolymerizible alignment material for liquid crystal alignment include those prepared by photodimerization such as cinnamate, coumarin, and chalcon, by photoisomerization of polymer having an azobenzene group, and by photodegradation of polyimide polymer. However, these materials show poor thermal stability or light stability, and contamination due to byproducts may occur.
To manufacture a retardation film, a view angle compensating film, a brightness improving film using a polymeric liquid crystal compound, an alignment layer is generally formed on a plastic substrate. However, even though using the composition for alignment layer prepared by the above process, its application is limited to the types of plastic substrate.